Highly stable opto-mechanic switches

ABSTRACT

The present invention provides improved optical switches in which only a spatial beam shifting of a small free space offset is required to direct optical pathways between plural fiber ports. This is achieved by spacing two fibers closely and collimating their beams with one imaging lens for compactness. Advantageously, the inventive switches incorporate beam correcting devices to render the beam propagations parallel, allowing light beams to be efficiently coupled into two fibers that sharing a single lens with substantially improved stability.

FIELD OF THE INVENTION

[0001] The present invention is related to optical devices; moreparticularly, the invention relates to mechanical optical switches.

BACKGROUND OF INVENTION

[0002] Optic switch is a device for directing optical signals alongselected fibers of an optic network, in which light signals aretransmitted along optical fibers to transfer information from onelocation to another. The desirable optical switch performance include:high speed switching, low optical insertion loss, good repeatability,long operation lifetime, small size, and low cost. Optical switch is akey component in today's optic network, analogous to the electricalswitches in electrical networks. However, it has not been widely adoptedbecause its lack of reliability and its high cost associated with itsfabrication difficulty.

[0003] Mechanical fiber optic switches using movable light guidingelements for alternating optical beam paths to effectuate switching arethe dominant optical switching component used in currenttelecommunication systems. This is because, in comparison with othermeans, mechanical optical switches are simpler in construction andproduce smaller distortions to the passing optical signals. For example,current non-mechanical switching technologies are based on changes ofeither optical phase or polarization. Consequently they have intrinsicdrawbacks of polarization and wavelength dependences and induce signaldistortions that become problematic as the channel count andtransmission speed increase in the new generation network systems.

[0004] In an optical switch, light signal must be accurately enteredinto an optical fiber, or much of the signal strength will be lost. Thealignment requirements of modem single mode optic fibers areparticularly stringent, as their core diameters are typically as smallas 2 to 10 micrometers and their acceptance angle is fairly narrow. Foroptical switches, alignment and maintenance of precision optical pathshave been the main technical difficulty, since a slight misalignment cancause large insertion losses. Therefore, the cost and reliability ofmechanical optical switches are primarily determined by the fiberalignment/package design.

[0005] Prior mechanical optical switch designs incorporate fibercollimator lenses, such as Grade Refractive Index (GRIN) lens, toincrease the alignment tolerance to some extend. The collimator lensesenlarge the optical beams at least ten to one-hundred times larger sothat insertion losses will be minimized when there is a few micrometersof misalignment between the light path from the input fiber to theoutput fiber. However, the use of individual collimator lensesdisadvantageously increases the separation between adjacent fibers,resulting in the need for large beam displacement for light pathswitching. Consequently, mechanical optical switches use individualfiber collimators have suffered from slow switching speeds and poorstability. An example of such an optical system is disclosed in U.S.Pat. No. 5,642,446 and No.

[0006] Recent version as described by Li et al. U.S. Pat. No. 6,215,919represents some improvement by using dual fiber collimator in which twofibers are placed next to each other and share a single imaging lens,substantially reducing the beam separation and overall device size. Adisadvantage of Li's switch is that the beam propagations are no longerparallel rather with an angle. Consequently the switch demands precisefabrication of a moveable prism with matching wedges that not onlyprecisely displace beam path but also must satisfy tight angulardeflection relations. As a result, this type of switch requires delicacyfor maintaining accurate alignment of each optic path, in which theangular and the spatial positions are interrelated. This type of switchis therefore often very difficult and costly to make and its operationis less stable.

[0007] For the above reasons, current mechanical optical switches areexpensive to produce and prone to fail with a short operation lifetime.

OBJECTS AND ADVANTAGES OF THE INVENTION

[0008] Accordingly, it is an object of the invention to provide a typeof mechanical optical switch that utilizes compact optical elements andis insensitive to both angular variations and position shifts of themoveable light-guiding element. The present inventive mechanical opticalswitches therefore provide critical advantages of switching a light beambetween an input fiber and output fibers with unprecedented stabilityand longevity against environment perturbations and wear-out.

[0009] It is a further object of the invention to provide a type ofmechanical optical switches, which require significantly, reducedalignment steps with large assembly tolerance that is suitable forlow-cost volume production.

[0010] The above objects and advantages, as well as numerousimprovements attained by the apparatus and method of the invention arepointed out below.

SUMMARY OF THE INVENTION

[0011] It is an object of the present invention to provide a compact andeconomical optical switch that can be efficiently coupled to pluraloptical fibers and these light couplings are less sensitive to themisalignment of the movable light guiding switching element. Theinvention consists of optical switches having at least three ports foroptical fibers. The inventive switches use at least one single lens tocoupling two fibers for compactness. The invention further consists of alight-bending device, situated to compensate for the angle between thetwo light beams that share the same lens, advantageously render themparallel. The inventive switches rely on spatially shifting lightpropagations of parallel beams, desirably increasing alignment tolerancethus stability and longevity.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012]FIG. 1 is a schematic view of a mechanical 1×2 switch according tothe principles of the present invention, and illustrates the arrangementof each element within the switch body for this first embodiment.

[0013]FIG. 2 is a schematic view of a mechanical 2×2 switch according tothe principles of the present invention, and illustrates the arrangementof each element within the switch body for this second embodiment.

[0014]FIG. 3 is a schematic view of a mechanical four-port wavelengthadd/drop switch according to the principles of the present invention,and illustrates the arrangement of each element within the switch bodyfor this third embodiment.

DETAILED DESCRIPTION OF THE SPECIFIC EMBODIMENTS

[0015] The inventive mechanical optical switches are based on animproved optical fiber packaging platform, wherein a closely spaced dualcollimator coupled with an angular correction prism forms a smallseparation and parallel beam propagation configuration, as shown inFIGS. 1, 2, and 3. This invention has several advantages over priormechanical optical switches. First, since two optical fibers on the sameside are placed next to each other, the resulting small beam separationsignificantly reduces the required traveling distance of the movingelement. Second, only one lens that is shared at least by one pair offiber is used on each side of the switch, leading to fewer opticalelements and a smaller footprint in comparison with conventionalswitches. Conventional optical switches have a bulky arrangement whereineach optical port has its own individual imaging element. Third, theincorporation of a beam angle correction system 7 and 8 compensates theangle separation between the two beams from the same imaging lens andadvantageously renders them parallel. The resulting parallel beamarrangement significantly increases the freedom thus the tolerance ofthe moveable switching light guiding element. Moreover, the advantage ofhaving an angular tuning element is that it substantially allows toadjust position and angular independently for maximum light coupling. Asa result, this inventive configuration greatly increases the switchingreliability, therefore, significantly increase the device operationallife time. Due to the loose tolerance requirement and independentposition and angular alignment, the inventive configuration also greatlyreduces the packaging difficulty, therefore, is particularly desirablefor volume production. The inventive optical switch's increasedtolerance further provides improvement in device thermal stability.

[0016] In one aspect of the invention, an optical signal may be reliablyswitched between two optical paths. In another aspect of the invention,a selected wavelength channel from data trunk can be reliably switchedbetween a pass-through state and an add/drop state in a four-portreconfugurable wavelength add/drop configuration. The inventive opticalswitches are a general light control device. The inventive opticalswitches may be used in telecommunications systems/subsystems, such asin WDM's, EDFA's, add-drop multiplexers, dispersion compensators,network switches, network protection and restoration, and OTDR's. Theinventive optical switch may also be used in other optical networks,such as sensors and computers. These and other advantages of theinventive optical switches are elaborated in the specific embodimentsnow described.

[0017] The present invention will be further described in terms ofseveral optical switch embodiments having specific components and havinga specific configuration. However, one of ordinary skill in the art willreadily recognize that this method and system will operate effectivelyfor other components having similar properties, other configurations,and other relationships between components.

[0018] To more particularly illustrate the method and system inaccordance with the present invention, refer now to FIG. 1, depicting afirst embodiment of a one-by-two fiber optic switch 100 incorporatingaspects of the invention. FIG. 1A depicts a light path from 1 to 3 andFIG. 1B depicts a light path from 1 to 2. The invention relates to anoptical switch comprising several optical components which are opticallycoupled along the longitudinal axis: a first fiber collimator 5 thatexpands and collimates light beam from fiber 1; an movable beam shifter9 for switching light path between 1 to 3 and 1 to 2; a beam angledeflector 7 which deflects all beams with a correction angle, such thatboth optical path are propagating parallel; a dual fiber collimator thatcollimates two beam with one shared imaging lens. The switch describedhere is a simple opto-mechanical device in which the light beam goes toone fiber without beam shift element 9 in the path and goes into anotherfiber when the beam shift element 9 is placed in the light path. Aconventional electrical actuator can be used to move the beam shifter inand out the beam path, consequently switches light from one port toanother port. Due to the improved design, the precision requirement ofthe actuator is substantially reduced. The following provides details ofthe drawings.

[0019] As shown in FIG. 1, a first optical fiber 1 is inserted into afirst glass capillary 5A. Fiber 1 emits a light beam that is collimatedby lens 5B. Optically aligned 5A and SB form collimator 5. Oppositefirst fiber 1, a second optical fiber 2 is inserted into a second glasscapillary 6A and a third optical fiber 3 is inserted into the same glasscapillary 6A adjacent to fiber 2. Light beam to or from fiber 2 andfiber 3 are collimated by lens 6B. Optically aligned 6A and 6B form adual collimator 6.

[0020] Lens 6B causes beams from port 2 and port 3 to make an angle θwith respect to the y-axis. A polarization-independent light-bendingdevice 7 is incorporated to correct this angle of propagation by benteach beam at an angle θ with respect to the y-axis. Therefore, lightbeam propagations from port 2 and port 3 become parallel to each otherafter device 7.

[0021] Considering a first switching state in which light path is from 1to 3, as indicated by solid beam propagation line in FIG. 1A. In thislight path sate, the light beam directly coupled to port 3 viacollimator 6. The movable beam displace element 9 is out of the lightpath.

[0022] Next, considering a second switching state in which light path isfrom port 1 to port 2, as shown in FIG. 1B by the dotted beampropagation line. Similarly, fiber 1 emits a light beam that iscollimated by lens 5B. The light beam is then passing through a beamdisplacement element 9 which displaces the beam by a distance thatmatches the parallel beam separation. Consequently an optical path fromfiber 1 to fiber 2 is established, as the beam displacement element 9 isplaced in the light path.

[0023] The general requirement for the beam shifter 9 is that it shiftsthe passing through light beam by a particular distance without makingangle deflection. One preferred embodiment of device 9 is using aparallel plate that is inclined with an angle in respect to the beampath, as shown in FIG. 1B. A light beam enters the parallel plate 9through a first facet and exits the plate through a second facet withoutundergoing any internal reflections. Beam displacement is accomplishedvia surface refractions due to index difference. The plate is orientedat an angle α to light beam propagation such that:$d = {\frac{L}{\cos \quad \left( {\arcsin \left( {{\frac{1}{n} \cdot \sin}\quad \alpha} \right)} \right)} \cdot {\cos\left( {\alpha - {\arcsin \left( {{\frac{1}{n} \cdot \sin}\quad \alpha} \right)}} \right.}}$

[0024] where d is the beam displacement that matches the parallel beamseparation, n is the refractive index of the plate, and L is the platethickness. From the above equation, it is straight forward to calculatethat the inventive design has a high stability. Wherein the beamdisplacement is independent of the spatial position of beam shifter 9,as long as it covers the beam size and wherein a large anglemisalignment in α of up to 1 degree does not produce substantial opticalcoupling loss due to the parallel diffraction design.

[0025] This beam deflector design offers an attractive feature in whichthe beam displacer 9 can be mass-produced by simply cutting a diskhaving two parallel surfaces of antireflection coating. Furthermore, inassembly, the angle α can be adjusted so that the beam displacement isprecisely matched with the beam separation of the dual collimator,achieving optimal optical coupling. Most importantly, the inventiveconfiguration substantially reduces the tight assembly tolerancerequirement that is often associated with conventional optical switches.This is because the optical loss is less sensitive to the slightmisalignment of the movable deflector 9. Conventional mechanical opticalswitches either require a large movable prism due to the large beamseparation such as in U.S. Pat. No. 6,215,919, or require a complexmoveable prism which must simultaneous maintains precise beamdisplacement and tight angular deflection relations such as in U.S. Pat.No. 6,215,919.

[0026] Device 9 is made of a parallel plate of a homogenous transparentsolid. Large index of reflection is preferred to reduce the beam shiftersize. One example is polycrystalline ZnSe which is isotropic and has alarge index refraction of 2.4 at the communication mid IR band.

[0027] One preferred embodiment of device 7 consists of a tapered glassprism, whose angle is adjusted so that beams enter from fiber port 2 or3 are rendered parallel to the y-axis as the beams exit device 7. Onespecific embodiment of a roof glass prism is illustrated in FIGS. 1, 2,and 3. Other shapes and constructions of prisms can also perform thesame function.

[0028] Referring to the FIG. 2, there is shown a second embodiment of atwo-by-two fiber optic switch 200 incorporating aspects of theinvention. Switch 200 utilizes the optical elements described above andthe same reference numerals are used to refer to the same parts. Theswitch 200 has four optical fiber ports, a first fiber 1 and a fourthfiber 4 input light beams through a dual collimator 5 and a second fiber2 and a third output fiber 3 receive the light beams through a dualcollimator 6 that is placed opposite to collimator 5. The two lightbeams from the dual collimator have an angle with respect to each otheror two light beams need an entry angle in order to optimally couple intothe dual collimator. In the inventive design, a light-bending device 7and 8 are incorporated to correct the angle separations for collimator 5and collimator 6, respectively. Therefore, light beam propagationsbetween device 7 and device 8 become parallel.

[0029] As is apparent from the FIG. 2A, the switch is aligned such thatthe light beam from fiber 1 propagates along free beam path 11 andenters fiber port 3 and the light beam from fiber 4 propagates alongfree beam path 12 and enters fiber 2, forming one switching state. Whena beam displacer 10 is placed in beam path 11 and beam path 12, the twolight paths exchange position, becoming beam 13 and beam 14,respectively. Therefore, the light beam from fiber 1 is guided to fiber2 and the light beam from fiber 4 is guided to fiber 3, achievingtwo-by-two optical switching.

[0030] One embodiment of beam displacer 10 is a rhombic-like glassprism, as shown in FIG. 2, which is based the same in design principleas that of beam displacer 9 in the first switching embodiment. Displacer10 essentially combines two parallel plates 9 into one. Instead of tworefraction surfaces in device 9, device 10 has four refractive surfacesto displace two beams in opposite direction. Therefore, device 10exchanges the optical paths of two parallel beams by means of fourrefractions at their entry and exit points and still maintain the highstability advantages as described in the first embodiment. Similarly,the prism comprises a homogenous transparent solid, preferably havinglarge index refraction, such as ZnSe. There is a plane of symmetrythrough the prism such that the one face of the first pair that containsthe entry point and the other face of the second pair that contains theexit point each form an angle α. All opposite planar faces are parallelto each other. As discussed in embodiment 1, this preferred beamdisplacer design has an advantage wherein the switch 200 optical lossesis insensitive to a small perturbation in the prism alignment bothangular and spatial. The prism's geometry minimizes the size of theprism and the shift distance to the prism's activated position.

[0031] Referring to the FIG. 3, there is shown a third embodiment of amechanical wavelength add/drop fiber optic switch 300 incorporatingaspects of the invention. Switch 300 utilizes the optical elementsdescribed above and the same reference numerals are used to refer to thesame parts. The switch 300 has four optical fibers, a first fiber 1 anda fourth fiber 4 couple light beam through a dual collimator 5 and asecond fiber 2 and a third fiber 3 couple light beam through a dualcollimator 6 that is placed opposite to collimator 5. The two lightbeams from the dual collimator have an angle with respect to each otheror two light beams need a separation angel in order to optimally coupleinto the dual collimator. In the inventive design, a light-bendingdevice 7 and 8 are incorporated to correct the angle separations forcollimator 5 and collimator 6, respectively. Therefore, light beampropagations become parallel between device 7 and device 8.

[0032] As shown in FIG. 3, an input beam 30 from fiber 1 that containsthe full spectrum of data (λ_(l) to λ_(n)) reaches thin-film opticalfilter 15 and is thereby separated into a passing through beam 32 of aselected wavelength band λ_(x) and a reflected beam 31 containing therest of wavelength bands. This reflected beam is permanently coupledinto the output fiber 4.

[0033] Considering a first bypass switching state in which light path ofthe selected spectral band is from port 1 to port 4, as indicated inFIG. 3B. In this light path, light beams 32 is reflected back by a rightangle prism which is electrically actuated to block the beam. Light beam32 is efficiently coupled into fiber 4 through the angle corrector 8.Therefore, in this bypass switching state of operation, the incomingoptical signal continuously flows through the inventive device with thefull spectrum of data.

[0034] Next, considering a wavelength add/drop operation state in whichlight path for the filtered spectral band λ_(x), beam 32, is from port 1to port 2 and light path for the substitute add signal of λ_(x), beam33, is from port 3 to port 4, as indicated in FIG. 3B. In this lightpath sate, in which the moveable prism 16 is out of the light path, thefiltered beam 32 is coupled into fiber 3 via angular correctors 8 and 7.Simultanously, beam 33 that is entered through fiber 3 is also focusedinto fiber 4, as shown in FIG. 3B. Therefore an optical path from fiberport 1 to fiber 2 for the selected wavelength band λ_(x) is establishedand at the same time light path for add optical signal from port 3 toport 4 is also established. In another words, a pre-defined opticalchannel is dropout from the incoming optical data stream and the addsignal is simultaneously substituted back into the output data stream.

[0035] The general requirement for device 16 is a compactretro-reflector. One embodiment of device 16 is using right angel prism,as shown in FIG. 3. Right angle prism is easy to make and provides goodcoupling efficiency and stability against small position deviations,commonly associated with mechanical actuators.

[0036] The above descriptions of three switch embodiments are veryspecific examples. It will be apparent to a person of average skill inthe art that many variations of the switch are possible within the scopeof the invention. Accordingly, the scope of the invention should bedetermined by the following claims and their legal equivalents.

What is claimed is:
 1. An optical switch for directing light from afirst fiber to a second fiber or to a third fiber, said second fiber andsaid third fiber being located adjacent to each other along alongitudinal axis and opposite said first fiber along said longitudinalaxis, said optical switch comprising along said longitudinal axis insequence from said first fiber to said second and third fibers: a) afirst lens for guiding light from said first fiber and to said second orthird fibers; b) a beam shifter positioned in said free beam path forshifting said light beams by an offset; c) a means for moving said beamguiding element in and out of said free beam path; d) a beam correctorfor bending said beam with an angular θ in respect to said longitudinalaxis; e) a second lens for guiding light to said second or third fiberfrom said first fiber, wherein light passing from said second fiber orthird fiber exits said second lens at an angle 0 with respect to saidlongitudinal axis.
 2. The optical switch of claim 1 wherein said angle θis between 1° and 4°.
 3. The optical switch of claim 1 wherein said beamshifter comprises a parallel plate of a homogenous transparent solid. 4.The optical switch of claim 1 wherein said beam shifter is made of largeoptical indexes of reflection polycrystalline materials of ZnSe and ZnS.5. The optical switch of claim 1 wherein said beam corrector is a glassprism that provides a beam a receiving angle for fiber in dual fibercollimator.
 6. The optical switch of claim 1 wherein said beam correctorcomprises two tapered birefringent plates that provides a beam areceiving angle for fiber in dual fiber collimator.
 7. A two-by-twooptical switch for coupling light among four fibers, a first fiber and afourth fiber being located adjacent to each other along a longitudinalaxis, and a second and a third fiber being located adjacent to eachother and opposite said first fiber and forth fiber along saidlongitudinal axis, said optical device comprising along saidlongitudinal axis in sequence from said first fiber and fourth fiber tosaid second and third fiber: a) a first lens for guiding light beamsfrom and to said first and forth fibers; whereby said first and forthfibers share a single said lens, wherein light passing from said firstand forth fibers exit said first lens at an angle θ₁ with respect tosaid longitudinal axis; b) a first light corrector for correcting saidbeam angular separation θ₁ for both said fiber 1 and fiber 4 such thatthe said two light beams become parallel; c) a beam shifter positionedin said free beam path for exchanges the optical paths of the said twoparallel beams; d) a means for moving said beam shifter in and out ofsaid free beam path; e) a second light guiding device for correctingbeam angular separation between said fiber 2 and fiber 3 such that thesaid two light beams propagate with an angle θ₂ with respect to saidlongitudinal axis; f) a second lens for guiding light beams from and tosaid second and third fibers; whereby said second and third fibers sharea single said lens; wherein light passing from said second and forthfibers exit said first lens at an angle θ₂ with respect to saidlongitudinal axis.
 8. The optical switch of claim 7 wherein said angleθ₁ and θ₂ are between 1° and 4°.
 9. The optical switch of claim 7wherein said beam correctors are glass prisms that provide a beam areceiving angle for fiber in dual fiber collimator.
 10. The opticalswitch of claim 7 wherein said beam shifter comprises a homogenoustransparent solid prism with two sets of parallel surfaces arranged atan angle such that the said beam shifter exchanges the optical paths oftwo parallel beams.
 11. The optical switch of claim 7 wherein said beamshifter is made of large optical indexes of reflection polycrystallinematerials of ZnSe and ZnS.
 12. An optical wavelength add/drop switch forcoupling light of selected wavelength band among four fibers, a firstfiber and a fourth fiber being located adjacent to each other along alongitudinal axis, and a second and a third fiber being located adjacentto each other and opposite said first fiber and forth fiber along saidlongitudinal axis, said optical device comprising along saidlongitudinal axis in sequence from said first fiber and fourth fiber tosaid second and third fiber: a) a first lens for guiding light beamsfrom and to said first and forth fibers; whereby said first and forthfibers share a single said lens, wherein light passing from said firstand forth fibers exit said first lens at an angle θ₁ with respect tosaid longitudinal axis; b) a thin film optical filter for passing aselectively wavelength band from said fiber 1 and reflecting light beamcontaining the rest of wavelength bands into said fiber
 4. c) a firstlight corrector for correcting passing beams with an angular θ₁ for bothsaid fiber 1 and fiber 4 such that the light beams from said fiber 1 andsaid fiber 2 become parallel; d) a beam reflector positioned in saidpassing beam path for reflecting light beam from said fiber 1 back intosaid fiber 4; e) a means for moving said beam reflector in and out ofsaid passing beam path; f) a second light guiding device for correctingbeam angular separation between said fiber 2 and fiber 3 such that thesaid two light beams propagate with an angle θ₂ with respect to saidlongitudinal axis; g) a second lens for guiding light beams from and tosaid second and third fibers; whereby said second and third fibers sharea single said lens; wherein light passing from said second and forthfibers exit said first lens at an angle θ₂ with respect to saidlongitudinal axis.
 13. The optical switch of claim 7 wherein said angleθ₁ and θ₂ are between 1° and 4°.
 14. The optical switch of claim 7wherein said beam correctors are glass prisms that provide a beam areceiving angle for fiber in dual fiber collimator.
 15. The opticalswitch of claim 7 wherein said beam reflector is a right angle reflectorof a homogenous transparent solid.